A generator or a circuit breaker for a distribution network constitutes an application of such an electrical apparatus, in which the dielectric gas contained inside the case is sulfur hexafluoride SF.sub.6, for example. SF.sub.6 at a pressure of a few bars, either in the pure state or mixed with some other gas such as air or carbon tetrachloride CCl.sub.4, is used to extinguish an electric arc that forms inside the case when the circuit breaker is opened.
The pressure sensor and the temperature sensor are mounted from the outside on the case and they communicate with the dielectric gas via a duct passing through the thickness of the case. Each record of pressure and temperature as measured simultaneously by the sensors can be used to calculate a density value that is representative of the real density of the dielectric gas contained in the case, to within the uncertainty of measurement error.
It is known that by measuring temperature simultaneously with pressure it is possible to apply temperature compensation to take account of the way pressure can decrease merely because the dielectric gas has contracted under the effect of a decrease in temperature, without there being any loss of mass or leakage of the dielectric gas from the case.
With the above-described method, it is possible to monitor mass losses of a few percent per annum. However, in the future, standards concerning protection of the environment may impose monitoring for leakage at a rate of one part per thousand per annum, particularly in respect of SF.sub.6 or CCl.sub.4.
On its own, temperature compensation turns out to be incapable of providing monitoring to that degree of accuracy, given that the pressure value as measured is corrected as a function of the temperature value as measured and not as a function of the real value of the temperature of the dielectric gas contained inside the case.